1. Field of the Invention
The present invention relates to a glass antenna for a vehicle in which, on or in a window glass including a defogger having a plurality of heater wires that run in parallel, first and second antenna conductors, and first and second feeding portions that are adjacent to each other in the direction that is parallel to the parallel running direction of the plurality of heater wires are disposed, and a window glass for a vehicle including the glass antenna.
2. Description of the Related Art
Conventionally, as means for eliminating variation (fading) of the reception level of a radio wave due to interference between a direct wave and a reflected wave reflected from an obstacle such as a mountain or a building, for example, the diversity system is known as disclosed in JP-A-6-21711. In the automobile antenna apparatus disclosed in JP-A-6-21711, a main antenna which receives an FM broadcast, and which outputs an FM main signal, and a sub antenna which receives an FM broadcast, and which outputs an FM sub signal are disposed in a backlite of an automobile. The FM main signal and the FM sub signal are synthesized with a predetermined phase difference. When the level of synthesis is lower than a predetermined value, the phase difference is changed so as to obtain a signal level sufficient for reception. Namely, the level of synthesis is changed by adjusting the phase difference in the synthesis.
Usually, it is known that, by means of increasing the spatial distance between a plurality of antennas in accordance with the wavelength of a radio waves to be received, received signals of the radio wave which are received respectively by the antennas are theoretically not correlated with one another, and the so-called spatial diversity effect is obtained. Namely, as the distance between a plurality of antennas is further increased, it is possible to further decrease the correlation coefficient indicating the degree of correlation between the amplitude variation of a received wave which is received by one of the antennas, and that of a received wave which is received by the other antenna. Therefore, the spatial diversity effect can be sufficiently exerted.
In a glass antenna which is formed on a window glass, however, the physical distance between antennas cannot be measured unlike a pole antenna, and hence it is difficult to design the antenna based on the spatial distance. Therefore, the assignee of the present invention has found that, in the case of a glass antenna in which two antenna conductors are disposed on a window glass for a vehicle, when a radio wave of a constant frequency is transmitted, the spatial diversity effect can be more sufficiently exerted on the glass antenna as the phase difference δ produced between a received wave which is received by one of the antenna conductors, and that which is received by the other antenna conductor is larger. Namely, the phase difference δ can be deemed to be equivalent to the inter-antenna distance.
In order to sufficiently obtain a requested spatial diversity effect, therefore, the phase difference δ which is detected as the characteristics of a glass antenna itself must be increased by tuning the placement positions of antenna conductors, the shapes of the antenna conductors themselves, or the like. When the placement positions of feeding portions respectively for two antenna conductors are separated from each other, for example, also the placement positions of the two antenna conductors can be easily separated from each other, and hence the phase difference δ is liable to be increased.
However, there is a case where feeding portions respectively for two antenna conductors are restricted to be close to each other by request of the specification of a vehicle such as the placement positions of the feeding portions, and wiring locations. In this case, it is difficult to increase the phase difference δ.